A lithium secondary battery using graphite as a negative electrode has been studied extensively because it is free from the old problem accompanying the use of metallic lithium or the problem of dendrite growth at the time of charging, it generates high electromotive force and it gives a highly reliable performance.
Carbonaceous materials classified as graphite are diversified in shape, structure and texture and the differences in these factors are reflected in the performance of electrodes. A secondary battery in which flake graphite such as natural graphite is used as a graphite material is known to have a higher discharge capacity than the one in which synthetic graphite obtained by graphitizing mesophase microbeads, pitch-derived carbon fibers, pitch coke and the like is used because of a highly developed crystal structure of flake graphite. However, one problem with flake graphite is a high initial irreversible capacity of a battery made therefrom. In addition, it appears difficult to obtain a high discharge capacity when flake graphite is used under the conditions of large charge and discharge current. Further problems associated with flake graphite are high bulk density of particles and difficulty in preparing a slurry for coating use. For the purpose of improving the maneuverability and particle characteristics of flake graphite, processes consisting of blending flake graphite with an organic binder or coating the surface of flake graphite with an organic binder followed by granulation and heat treatment are proposed in JP10-36108 A and elsewhere. According to these processes, however, a part of the binder component remains behind thereby reducing the discharge capacity.
When mesophase microbeads collected at the stage in which an optically anisotropic phase forms spherically in mesophase pitch are used as a raw material for graphite, the growth of carbon layer after graphitization is inferior to that in natural graphite and the discharge capacity thereby obtained amounts to merely 80–85% of the theoretical discharge capacity.
It is reported in J. Electrochem. Soc., Vol. 142, No. 8, 2564 (1995) that pitch-based carbon fibers possess a variety of properties as synthetic graphite and yield electrodes of excellent heavy load characteristics. Being a fiber, however, raises problems such as the following; the growth of crystal structure is retarded to keep the discharge capacity from increasing beyond a moderate level and the necessity of installing a fiber-making step and the like incurs an extra manufacturing cost.
Pitch coke belongs to a class of readily graphitizable materials and, upon graphitization in an ultra-high temperature range, may form carbon network layers whose interlayer distance is close to that of natural graphite; however, in the case where pitch coke has not much of optically anisotropic textures, graphitization does not progress to a degree comparable to natural graphite and the carbon network layers are not preferentially oriented in the specified direction in the crystal structure. In consequence, graphite of this kind is free of the restriction on the current density such as seen in the case of natural graphite like flake graphite and is an extremely promising material for negative electrode of lithium secondary battery as evidenced by a large number of researches made on it up to the present (for example, JP63-121257A, JP1-204361A and JP4-206276A). However, a material obtained by calcining ordinary pitch coke at an ultra-high temperature (2000–3000° C.) shows a lower discharge capacity (<300 mAh/g) than the theoretical discharge capacity (372 mAh/g).
An attempt has been made to provide graphite with a high discharge capacity by adding boron to carbon particles derived from pitch coke or pitch, for example, in JP10-223223A This method is effective for raising the discharge capacity and reducing the irreversible capacity and provides a technique effective for improving the performance of graphite materials for negative electrodes. On the other hand, this method requires a pulverizing operation in the step for particle size control and may develop the possibility of deteriorating the particle characteristics such as reducing the bulk density and tap density and increasing the specific surface area. In addition, there remain practical problems in need of solution such as lack of stability in manufacture of electrode foils and insufficient particle characteristics relating to cycle performance of materials for negative electrodes.